I was hoping that someone would have some advice for me. I have three dogs, two neutered males that I have had together for a while and never had any problems with. But recently we got another dog who has become completely obsessed with one of the dogs. The dog we got recently is an un-neutered 11 year old Shepard/lab/rottie mix. At first he was humping our pit/lab mix and following him around and we thought it might be a dominance thing and that they would figure it out and get over it. but we got him in November and it has only gotten worse. He seemed to calm down for a while but then out pit/lab mix got a bath and after the Shepard mix wouldn't leave him alone. He constantly follows him around, humping him and licking his genitals. If he isn't near him he will wine and bark. It is coming to the point where the pit mix is trying to get away from him and hide. It is also only the pit mix that he seems to be obsessed with, we have a neutered yellow lab as well and he doesn't bother him at all. Any advice would be greatly appreciated.

Unfortunately one of the fairly common side effects of castration can be that the castrated male is attractive to other males like a bitch in season, especially if the dog was castrated young.

Another possibility is infection which to an inexperienced male especially, can smell like a bitch in season too.  Could your 'attractive' male have an anal gland infection, and infected sore etc?